Seems I recently saw a notification that Yamaha has begun to lubricate its MotoGP gearboxes with something other than engine oil. If true, there could be a couple of reasons for this.
In four-stroke motorcycle engines, it seems like a useful simplification to lube the gearbox with engine oil. After all, the oil is there, and there is already an oil pump. Just run a squirt into each mesh and some into the shafts to lube the free-spinning gears.
When I began to see such engines apart, I noticed that their gears were of unusually large diameter (as compared with the more compact two-stroke racing gearboxes I was familiar with). While oils specifically for gear lubrication contain extreme pressure additives, engine oil cannot (extreme pressure additives and ignition/combustion aren’t the best of buddies). As a result, gear teeth lubed with engine oil must be operated at reduced tooth-to-tooth pressure to avoid scoring and failure. To transmit high power through such reduced pressure, the gears themselves are made larger in diameter, and their shafts are spaced farther apart, giving the limited tooth pressure more leverage.
But bigger gears are not only heavier, they’re bulkier. And since every team uses any means to move its swingarm pivot farther forward, smaller-diameter gears separately lubricated by gear oil might offer useful geometric possibilities.
Another reason for separate lubrication of the gearbox might be this: Yamaha’s version of a seamless-shift gearbox has special needs (unknown to us) that cannot be provided by engine oils of modern (watery) low viscosity.
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